The present invention relates generally to industrial controls, and pertains more particularly to robust, easy to configure, and water-resistant interconnecting devices for use with sensors.
In industrial control systems, a programmable logic controller (PLC) or computer is often required to send output control signals to, and receive input sensor signals from, a number of input/output (I/O) points in the system. Typically the connection to each I/O point consists of three wires: a wire which carries a ground signal, a wire which carries a power signal, and a wire which carries an I/O signal which either is a control signal that is output from the controller to external circuitry, or a sensor signal from the external circuitry that is input to the controller.
The wiring interconnections from the I/O points to the PLC are typically made at a termination block. At the termination block, the ground signals and the power signals connected to the different I/O points are combined respectively. The termination block passes the control signals and the sensor signals to the programmable logic controller.
The three individual wires from each sensor or control, grouped together in a cable, have typically been connected to a terminator block using wire leads held in individual wire receptacles. However, this mechanical arrangement frequently results in the sensor cables being subject to repetitive motion. Over the course of time, this can result in the wires within the cables becoming intermittently or permanently defective. In addition, replacement of I/O cables with wire leads held in individual wire receptacles can be time consuming.
To alleviate this problem, some manufacturers have utilized I/O cables with connectors which can be plugged into a termination block. This has provided for an easier replacement of defective cables. However, existing termination systems which utilize these types of connectors typically cannot be used in an environment where water or other liquids can splash onto the termination block, much less in an environment requiring that the cables and termination block be submersed in water or other liquids.
In addition, sensors are typically designed to provide an output signal in one of two ways, either by sourcing or sinking current. A PNP sensor that sources current (for example, one that uses a PNP transistor for its output stage) can directly interface with a PLC input which sinks current, while an NPN sensor that sinks current (for example, one that uses an NPN transistor for its output stage) can directly interface with a PLC input which sources current. Since a PLC typically packages in a single I/O module a number of input lines all of which must operate in either a sourcing or a sinking mode, the I/O module can directly connect only with a group of sensors which are of the same type (either all NPN or all PNP respectively). If some sensors of the incompatible current type are to be connected to the same I/O module (eg. an NPN sensor to a sinking PLC input, or a PNP sensor to a sourcing PLC input), additional circuitry must be added, generally at the termination block, in order for the PLC to properly read the sensor output.
The most common additional circuitry consists of pull-up or pull-down resistors. These resistors are generally inexpensive, but they are difficult to wire to the termination block in a robust and reliable way, as is needed in most industrial environments. In addition, the PLC program which reads the sensor must invert the sense of the logic level. For example, if an NPN sensor is connected to a sinking PLC input using a pullup resistor, and the sensor generates a logic level xe2x80x9c1xe2x80x9d signal (corresponding to the sensor turning on), the PLC input will detect a logic level of xe2x80x9c0xe2x80x9d, corresponding to the sensor being xe2x80x9coffxe2x80x9d. Therefore, the PLC program will have to invert the signal and recognize it as a logical xe2x80x9c1xe2x80x9d instead. This undesirably adds to the complexity of the PLC program, and reduces the clarity of its logic. Also, discrete resistors wired to termination blocks are not splash-resistant or submersible.
Another type of additional circuitry that allows connecting mismatched sensor and PLC input types are solid-state relays. While solid-state relays do not require any logic sense inversion in the PLC program, they are more expensive than resistors. Typically they must be wired to the sensor and PLC input at the termination block using discrete wires; therefore, they do not form a splash-resistant or submersible connection to the system. In addition, skill in reading technical documentation is required to understand how to properly wire each particular type of solid-state relay into the system so that it operates in the desired manner.
Accordingly, what is still needed is an easy to use, intuitive, mechanically robust, and programmatically transparent solution for connecting NPN sensors to current sinking inputs of a programmable logic controller, and connecting PNP sensors to current sourcing inputs of a programmable logic controller. Such a solution would also be applicable in a wide variety of markets if it could be conveniently packaged in a manner that is splash-resistant or even allows the connections to be immersed in liquid.
In a preferred embodiment, the present invention provides a sensor output inverter that can be connected between a sensor and a controller in order to allow NPN sensors to be used with current sinking controller inputs, and PNP sensors to be used with current sourcing controller inputs. The inverter is preferably packaged in a mechanically robust, plug-together arrangement that provides a programmatically transparent connection of the sensor to the controller. The inverter has a first optoisolator with its collector connected to a reference voltage, and a second optoisolator with its emitter connected to a ground voltage. A voltage divider for biasing the optoisolators is connected between the reference and ground voltages, and a threshold voltage point of the divider is connected to the anode of the first optoisolator and the cathode of the second optoisolator. The preferred voltage divider has an 8.25 kilohms first resistor connected between the reference voltage and the threshold voltage point, and a 3.48 kilohms second resistor connected between the threshold voltage point and the ground voltage.
The signal input of the inverter is connected to the cathode of the first optoisolator and the anode of the second optoisolator, while the signal output is connected to the emitter of the first optoisolator and the collector of the second optoisolator. In operation, a signal input equal to the ground voltage produces a signal output equal to the reference voltage, and vice-versa. The inverter works with reference voltages of between ten volts and thirty volts; the preferred reference voltage is twenty-four volts. The preferred inverter has a female DIN-style input connector with a protruding beveled flange and pins for the reference voltage, ground voltage, and signal input, and a male DIN-style output connector with pins for the reference voltage, ground voltage, and signal output. When assembled, the input connector is located at one end of a housing, and the output connector at the other end of the housing. The electronic components are mounted on a printed circuit board which is mounted to and electrically connected to the input connector and the output connector. The board has a thickness such that it forms a good mechanical fit edgewise between the pins of the input connector and output connector, which are mounted in-line and facing in opposite directions, and the pins are preferably soldered to surface-mount contact pads on the printed circuit board. The housing has a first end with a first opening opposite an second end with a second opening. The first opening is sized to admit one connector, typically the female, while the second end has a reducing beveled lip which makes the second opening is smaller than the first opening such that the beveled lip engages the protruding beveled flange of the female connector when the inverter is assembled.
An alternate embodiment of the termination system includes a submersible termination block having a sealed housing having a number of sensor connectors and a data connector. Each sensor connector can mate with a sensor, a signal cable, or a sensor output inverter so as to form a splash-resistant and submersible seal. Each sensor connector has a recessed beveled flange which frictionally engages a mating connector having a single barbed flange so as to provide the seal. A data cable having a corresponding signal line for the signal lines in each of the sensor connectors makes a similar seal with the data connector. Since the sensor output inverter is typically potted or overmolded, and uses the same style connectors, a termination system including connected inverters is likewise submersible.
The present invention may also be implemented as a method for providing a sensor signal to a controller. A sensor output inverter is provided which has an inverter input and an inverter output that either both source current or both sink current. The inverter input is connected to a sensor having a sensor output which operates in the opposite manner (for example, if the inverter input and inverter output sink current, the sensor output sources current), and the inverter output is connected to a controller having a controller input which operates in the same manner as the sensor output (for example, the controller input sources current). Connecting the inverter between the sensor and the controller allows the two to operate together, even though they could not be directly connected to each other. The inverter input may be plugged directly into the sensor output, or a sensor cordset may be connected between the inverter input and the sensor output. The inverter output may be connected to the controller input by first connecting the inverter output to one of the sensor connectors on a termination block, and then connecting the data connector on the termination block to the controller; the inverter output may be plugged directly into the sensor connector, or a sensor cordset may be connected between the inverter output and the sensor connector.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate by way of example the principles of the invention.